Heat transfer tube having rare-earth oxide superhydrophobic surface and method for manufacturing the same

ABSTRACT

The present disclosure relates to a heat transfer tube having rare-earth oxide deposited on a surface thereof and a method for manufacturing the same, in which the rare-earth oxide can be deposited on the surface of the heat transfer tube to implement a superhydrophobic surface even under the high temperature environment and a plurality of assembled heat transfer tubes can be coated by coating a complex shape by depositing rare-earth oxide using a method for dipping a surface of the heat transfer tube and coating the same, thereby reducing or preventing the heat transfer tubes from being damaged during the assembling of the heat transfer tubes after the coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/710,573 filed on Sep. 20, 2017, which claims priority to KoreanPatent Application No. 10-2016-0151124, filed on Nov. 14, 2016, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

Exemplary embodiments of the present disclosure relate to a heattransfer tube having a rare-earth oxide superhydrophobic surface and amethod for manufacturing the same, and more particularly, to a heattransfer tube having a rare-earth oxide superhydrophobic surface bydepositing a rare-earth oxide layer and a method for manufacturing thesame.

A nuclear power plant or a thermal power plant generates heat usinguranium, petroleum, coal, or the like as fuel to heat a watercirculating a system with the generated heat, thereby forming steam. Theformed steam rotates a turbine to produce electricity and the steampassing through the turbine is cooled in a condenser to be changed towater again. In particular, in a steam circulation power generationsystem, a water cooling type that performs cooling with water in acondensing process requires a large quantity of cooling water. Here, asthe cooling water used in the condenser, sea water is used. Therefore,to smoothly supply and discharge the sea water used as the coolingwater, the steam circulation power generation system is generallyinstalled near the coast.

In other words, the condenser is expressed as a steam condenser and thesteam condenser makes sea water continuously flow in a heat transfertube of the condenser to continuously turn a temperature of an innerwall of the condenser down. Then, the steam is cooled in the moment thatsteam discharged through a valve to rotate the turbine is directlybumped into the inner wall of the condenser and becomes condensed water(in a state in which steam returns to water) and the condensed wateragain returns to a boiler pipe into water of about 500° C. and passesthrough the turbine via the valve.

The boiler continuously makes hot water into supersaturated steam andbelches out the steam to the turbine through the valve and the steamcondenser continuously repeats a process of suddenly cooling the steamto return the steam to water again.

At this point, the cooling water cooling an outer wall of the condenserrequires a large quantity of water incomparable to that of the coolingwater cooling the mechanical friction heat, and sea water needs to becontinuously supplied while a power generator is operated.

The steam rotating the turbine contacts the inner wall of the condenser,and thus is cooled into water. At this point, to increase the quantityof steam contacting the inner wall of the condenser, a plurality of heattransfer tubes are included to increase a contact area.

The condenser may corrode due to condensation at an outside of the tube,and may corrode due to a condensed fluid remaining on a surface thereof,and the like. Similarly, even in the case of a heat exchanger used in apower plant, the condenser may corrode due to the condensation at theoutside of the tube when heat is exchanged between channels crosslypassing through a heat transfer plate, and corrode due to the condensedfluid remaining on the surface thereof, and the like.

One attempt to address this problem is a cross-linked repellent thinfilm that includes resin having a fluorine atom containing group;quaternary ammonium group-containing modified epoxy resin; and aminoresin. However, the repellent thin film has a problem in that it isdifficult to form a superhydrophobic thin film in which a contact anglebetween the surface thereof and a water drop is equal to or more than150° and it is difficult to hold repellent coating even under a hightemperature environment.

Therefore, an improved heat transfer tube and a method for manufacturingthe same capable of forming superhydrophobic thin film and of holdingrepellent coating even under the high temperature environment isdesirable.

BRIEF SUMMARY

An object of the present disclosure is to provide a heat transfer tubehaving a rare-earth oxide superhydrophobic surface and a method formanufacturing the same.

Another object of the present disclosure is to provide a heat transfertube capable of forming a superhydrophobic surface even under the hightemperature environment by deposing rare-earth oxide on a surface of theheat transfer tube and a method for coating a rare-earth oxide.

Still another object of the present disclosure is to provide a heattransfer tube capable of performing coating on a complex shape to coat aplurality of assembled heat transfer tubes by depositing rare-earthoxide using a method for dipping a surface of the heat transfer tube andcoating the same, thereby reducing or preventing the heat transfer tubesfrom being damaged during the assembling of the heat transfer tubesafter the coating and a method for manufacturing the same.

Other objects and advantages of the present disclosure will be moreclearly described below with reference to the detailed description andthe claims.

Examples of the present disclosure are provided in order to morecompletely explain the present disclosure to those skilled in the art.Examples below may be modified in several different forms and does notlimit a scope of the present disclosure. Rather, these exemplaryembodiments are provided in order to make this disclosure more thoroughand complete and completely transfer ideas of the present disclosure tothose skilled in the art.

In addition, a thickness or a size of each layer will be exaggerated forconvenience of explanation or clarity and the same reference numberswill indicate the same components throughout the drawings. As used inthe present specification, a term “and/or” includes any one or at leastone combination of enumerated items.

Terms used in the present specification are for explaining theembodiments rather than limiting the present disclosure. Unlessexplicitly described to the contrary, a singular form includes a pluralform in the present specification. The word “comprise” and variationssuch as “comprises” or “comprising,” will be understood to imply theinclusion of stated constituents, steps, operations and/or elements butnot the exclusion of any other constituents, steps, operations and/orelements.

A heat transfer tube of the present disclosure means including a channelof a heat exchanger as well as a heat transfer tube configuring acondenser.

In accordance with one aspect of the present disclosure, there isprovided, a method for manufacturing a heat transfer tube having arare-earth oxide superhydrophobic surface, including: 1) preparing arare-earth oxide coating solution including Ce(NO₃)₃, peroxide, andwater; 2) sonicating the heat transfer tube; 3) dipping the sonicatedheat transfer tube of the step 2) into an acidic solution; and 4)dipping the heat transfer tube dipped into the acidic solution of thestep 3) into the rare-earth oxide coating solution of the step 1) toform a coating layer on a surface of the heat transfer tube, wherein thecoating layer includes the rare-earth oxide.

The preparing of the rare-earth oxide coating solution of the step 1)may include: 1-1) preparing a mixture by mixing the Ce(NO₃)₃, theperoxide, and the water; 1-2) sonicating the mixture of the step 1-1);1-3) agitating the mixture at 500 rpm for 10 to 30 minutes after thesonicating of the step 1-2); and 1-4) stabilizing the mixture for 50 to70 minutes after the agitating of the step 1-3) ends.

The method may further include: 5) performing a hydrocarboncontamination after the step 4).

The step 5) may include: 5-1) putting a container including alkene ofC₁₀₋₂₀ or an unsaturated fatty acid solution in an airtight container;5-2) putting the heat transfer tube dipped into the rare-earth coatingsolution of the step 4) in the airtight container of the step 5-1); and5-3) heating the airtight container of the step 5-2) in the oven of 40to 60° C. for 6 hours or more.

In the step 5), the coating layer formed on the surface of the heattransfer tube may further include a carbon coating layer.

The rare-earth oxide coating solution of the step 1) may include 4 to 9wt % of Ce(NO₃)₃, 1.3 to 2 wt % of peroxide, and the balance water.

The step 2) may include: 2-1) putting the heat transfer tube in acetoneand sonicating the heat transfer tube for 3 to 7 minutes; and 2-2) afterthe step 2-1), putting the heat transfer tube in ethanol and sonicatingthe heat transfer tube for 3 to 7 minutes.

The heat transfer tube of the step 2) may have a form in which aplurality of heat transfer tubes are assembled.

The acidic solution of the step 3) may be 2M of hydrochloric acid (HCl).

In the step 3), the heat transfer tube may be dipped in 2M ofhydrochloric acid (HCl) for 20 to 40 seconds.

In the step 3), a metal oxide layer formed on the surface of the heattransfer tube may be removed by dipping the heat transfer tube in the 2Mof hydrochloric acid.

The heat transfer tube may be made of copper or aluminum.

When the heat transfer tube is made of copper, in the step 4), the heattransfer tube may be dipped in the rare-earth oxide coating solution for20 to 40 minutes.

When the heat transfer tube is made of aluminum, in the step 4), theheat transfer tube may be dipped in the rare-earth oxide coatingsolution for 30 to 120 minutes.

The rare-earth oxide may be CeO₂.

A thickness of the coating layer may range from 100 to 400 nm.

In accordance with another aspect of the present disclosure, there isprovided a heat transfer tube having a rare-earth oxide superhydrophobicsurface, including: a coating layer formed on a surface of the heattransfer tube by the manufacturing method, wherein the coating layerincludes the rare-earth oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an SEM photograph of a heat transfer tube having a rare-earthoxide superhydrophobic surface according to the present disclosure;

FIG. 2 is a FIB photograph of the heat transfer tube having a rare-earthoxide superhydrophobic surface according to the present disclosure;

FIG. 3 is a SEM photograph of the heat transfer tube having a rare-earthoxide superhydrophobic surface according to the present disclosure;

FIG. 4 is a flow chart of a method for manufacturing a heat transfertube having a rare-earth oxide superhydrophobic surface according to thepresent disclosure;

FIG. 5 is a flow chart of a method for preparing a rare-earth coatingsolution according to the present disclosure;

FIG. 6 is a flow chart of a hydrocarbon contamination step according tothe present disclosure;

FIG. 7 is a diagram of the hydrocarbon contamination step according tothe present disclosure;

FIG. 8 is a photograph of a measurement result for a contact angledepending on a content range of wt % of Ce(NO₃)₃ in a rare-earth coatingsolution for manufacturing the heat transfer tube having a rare-earthoxide superhydrophobic surface according to the present disclosure;

FIG. 9 is a photograph of a measurement result for a contact angledepending on a content range of wt % of peroxide in the rare-earthcoating solution for manufacturing the heat transfer tube having arare-earth oxide superhydrophobic surface according to the presentdisclosure;

FIG. 10 is a photograph of a measurement result for a contact angledepending on a dipping time of the heat transfer tube made of copper tomanufacture the heat transfer tube having a rare-earth oxidesuperhydrophobic surface according to the present disclosure;

FIG. 11 is a photograph of a measurement result for a contact angledepending on a dipping time of the heat transfer tube made of aluminumto manufacture the heat transfer tube having a rare-earth oxidesuperhydrophobic surface according to the present disclosure;

FIG. 12 is a diagram of measurement equipment for measuring thermaldisclosure;

FIG. 13 is a photograph of an experiment result of measuring thermalstability;

FIG. 14 is a photograph of the experiment result of measuring thermalstability;

FIG. 15 is a photograph of the experiment result of measuring thermalstability;

FIG. 16 is a comparison photograph for measuring a change in contactangle due to a difference in form;

FIG. 17 is the comparison photograph for measuring the change in contactangle due to the difference in form;

FIG. 18 is the comparison photograph for measuring the change in contactangle due to the difference in form;

FIG. 19 is the comparison photograph for measuring the change in contactangle due to the difference in form;

FIG. 20 is the comparison photograph for measuring the change in contactangle due to the difference in form;

FIG. 21 is an SEM photograph and an FIB comparison photograph of asample due to a difference in a manufacturing method for coatingrare-earth oxide;

FIG. 22 is the SEM photograph and the FIB comparison photograph of thesample due to the difference in the manufacturing method for coatingrare-earth oxide;

FIG. 23 is the SEM photograph and the FIB comparison photograph of thesample due to the difference in the manufacturing method for coatingrare-earth oxide; and

FIG. 24 is the SEM photograph and the FIB comparison photograph of thesample due to the difference in the manufacturing method for coatingrare-earth oxide.

DETAILED DESCRIPTION

Hereinafter, the present disclosure will be described in more detailwith reference to Example. These exemplary embodiments are to describein more detail and it will be apparent to those skilled in the art thatthe scope of the present disclosure is not limited to these exemplaryembodiments.

A heat transfer tube having a rare-earth oxide superhydrophobic surfaceand a method for manufacturing the same according to the presentdisclosure will be described in more detail with reference to FIGS. 1 to6.

FIG. 1 is an SEM photograph of a heat transfer tube having a rare-earthoxide superhydrophobic surface according to the present disclosure, inwhich the heat transfer tube made of copper and the heat transfer tubemade of aluminum are coated by being dipped in a rare-earth solution.Referring to FIG. 1, it may be confirmed that the rare-earth oxide iscoated on the copper or aluminum surface.

FIGS. 2 and 3 are FIB (focused ion beam) photographs of a heat transfertube having a rare-earth oxide superhydrophobic surface according to thepresent disclosure, in which the coating component and the thicknessrange of the coating may be confirmed. FIG. 2 is an FIB photograph forthe heat transfer tube made of copper, in which the rare-earth oxide iscoated, a thickness range ranges from 134 to 375 nm, and an averagethickness is 274 nm, but the present disclosure is not limited to theexample.

Further, FIG. 3 is an FIB photograph for the heat transfer tube made ofaluminum, in which the rare-earth oxide is coated, a thickness rangeranges from 144 to 210 nm, and an average thickness is 180 nm, but thepresent disclosure is not limited to the example.

It may be confirmed from FIGS. 2 and 3 that the heat transfer tube isdipped into the rare-earth coating solution to have the rare-earth oxidedeposited on a surface thereof, in which the coating layer has athickness ranging from 100 to 400 nm and an average thickness rangingfrom 100 to 250 nm and is made of the thin rare-earth oxide.

Referring to FIGS. 2 and 3, the heat transfer tube has rare-earth oxidecoated on the surface thereof and may further include carbon (C). Thatis, the surface of the heat transfer tube may include the rare-earthoxide and the carbon (C). When the surface of the heat transfer tube iscoated with only the rare-earth oxide, surface energy is increased andthus a contact angle is low, but a coating layer is added with thecarbon C having low surface energy, thereby increasing the contactangle. In more detail, when the coating layer of the surface of the heattransfer tube includes only the rare-earth oxide, the low contact angleappears due to a OH group present on the coating layer, but when thecarbon (C) is deposited, new bonding is formed and thus a high contactangle may appear, thereby implementing the superhydrophobic surface inthe heat transfer tube.

To manufacture the heat transfer tube having the superhydrophobicsurface according to the present disclosure, the assembled heat transfertube is dipped into the rare-earth coating solution to form the coatinglayer, but additionally, the heat transfer tube coated by being dippedinto the rare-earth coating solution is put in an airtight container andmay be heated in an oven of 40 to 60° C., and the airtight container mayinclude a container including alkene of C₁₀₋₂₀ or an unsaturated fattyacid solution, preferably, a container including 1-octadecene or oleicacid, but the present disclosure is not limited to this example.

Referring to FIG. 4, a flow chart of the method for manufacturing a heattransfer tube having a superhydrophobic surface according to the presentdisclosure includes: 1) preparing a rare-earth coating solution (S100);2) sonicating the heat transfer tube while putting the heat transfertube in acetone and ethanol (S200); 3) dipping the sonicated heattransfer tube into hydrochloric acid (S300); and 4) dipping the heattransfer unit into the rare-earth coating solution of the step 1)(S400).

The step 1 (S100) is a step of preparing a rare-earth coating solutionand in more detail, FIG. 5 is a flow chart of the method for preparing arare-earth coating solution according to the present disclosure, whichincludes 1-1) preparing a mixture by mixing Ce(NO₃)₃, peroxide(H₂O₂),and water (S110); 1-2) sonicating the mixture of the step 1-1) (S120);1-3) agitating the sonicated mixture (S130); and 1-4) stabilizing.

The step 1-1) (S110) is a step of mixing the Ce(NO₃)₃, the peroxide, andthe water, in which the water may be de-ionized water (DI water) but isnot limited thereto. In the step 1-1), in more detail, 4 to 9 wt % ofCe(NO₃)₃, 1.3 to 2 wt % of peroxide, and the balance water are mixed andwhen the Ce(NO₃)₃ is less than 4 wt % and exceeds 9 wt %, a recedingcontact angle was measured as less than 10° and even when the peroxideis less than 1.3 wt % and exceeds 2 wt %, the receding contact angle wasmeasured as less than 10°. That is, when wt % of the Ce(NO₃)₃ and theperoxide is out of the range, the surface of the heat transfer tube isnot uniformly coated, and therefore the phenomenon that a drop of watermay be pinned may occur.

When the mixture is prepared in the step 1-1) (S110), in the step 1-2)(S120), the sonication is performed for 15 to 25 minutes and after thesonication, an agitator agitates the mixture at 500 rpm for 10 to 30minutes and when the agitation ends, in the step 1-3) (S130), themixture suffers from the stabilizing for 50 to 70 minutes, therebypreparing the rare-earth coating solution.

When the rare-earth coating solution is prepared, the next step is thestep (S200) of sonicating the assembled heat transfer tube while puttingthe heat transfer tube in acetone and ethanol. In more detail, the heattransfer tube is sonicated for 3 to 7 minutes while being put in theacetone solution and then the heat transfer tube is sonicated for 3 to 7minutes while being put in the ethanol solution. Here, the sonication ofthe heat transfer tube in the acetone solution and the ethanol solutionis performed regardless of order.

The assembled heat transfer tube is sonicated in the acetone solutionand the ethanol solution and then 3) the heat transfer tube may bedipped into the hydrochloric acid (S300). As metal includes a metaloxide layer naturally generated, to remove the oxide layer naturallyformed on the surface of the heat transfer tube, the heat transfer tubemay be dipped into the hydrochloric acid, but the hydrochloric acid isonly an example and therefore any acidic solution that may remove themetal oxide layer naturally generated may be used.

The heat transfer tube dipped into the hydrochloric acid is dipped intothe rare-earth coating solution of the step 1) (S400) to coat thesurface of the heat transfer tube with the rare-earth oxide. In the step(S400) of dipping the heat transfer tube into the rare-earth coatingsolution, the dipping time may be different depending on whether theheat transfer tube is made of copper or whether the heat transfer tubeis made of aluminum. Preferably, when the heat transfer tube is made ofcopper, in the step 4) (S400), the heat transfer tube may be dipped intothe rare-earth coating solution for 20 to 40 minutes and when the heattransfer tube is made of aluminum, in the step 4) (S400), the heattransfer tube may be dipped into the rare-earth coating solution for 30to 120 minutes.

When the heat transfer tube made of copper may be dipped into therare-earth coating solution for less than 20 minutes and an excess of 40minutes, the receding contact angle is less than 10° and when the heattransfer tube made of aluminum may be dipped into the rare-earth coatingsolution for less than 30 minutes and an excess of 120 minutes, thereceding contact angle is less than 10°. That is, when the dipping timeof the heat transfer tube made of copper or aluminum into the rare-earthcoating solution is out of the range, the surface of the heat transfertube is not uniformly coated, and therefore the phenomenon that a dropof water may be pinned may occur.

FIG. 6 is a flow chart of a hydrocarbon contamination step of thepresent disclosure, in which as one embodiment of the presentdisclosure, in the step (S400) of dipping the assembled heat transfertube into the rare-earth coating solution, the surface of the heattransfer tube is deposited with the rare-earth oxide to form the coatinglayer, but the hydrocarbon contamination step (S500) is additionallyperformed and therefore the surface coating layer of the heat transfertube may be added with carbon (C).

In more detail, the step (S500) may include 5-1) putting the containerincluding the alkene of C₁₀₋₂₀ or the unsaturated fatty acid solution inthe airtight container; 5-2) putting the assembled heat transfer tubedipped into the rare-earth coating solution of the step 4) (S400) in theairtight container of the step 5-1); and 5-3) heating the airtightcontainer of the step 5-2) in the oven of 40 to 60° C. for 6 hours ormore.

FIG. 7 is a diagram of the hydrocarbon contamination step according tothe present disclosure, in which in the hydrocarbon contamination step,the container 100 including the alkene of C₁₀₋₂₀ or the unsaturatedfatty acid solution and the heater transfer tube 200 are put in theairtight container 300 and the airtight container is heated in the ovenof 40 to 60° C. for 6 hours or more.

EXAMPLE 1

Manufacturing of Heat Transfer Tube Coated with Rare-Earth Oxide

(1) Preparation of Rare-Earth Coating Solution

The mixture was prepared by mixing 4 wt % of Ce(NO₃)₃, 1.3 wt % ofperoxide, and 94.7 wt % of de-ionized water and the mixture wassonicated for 20 minutes. The sonicated mixture was sonicated at 500 rpmfor 20 minutes by the agitator and stabilized for 1 hour, therebypreparing the rare-earth coating solution.

(2) Coating of Heat Transfer Tube Made of Copper

The assembled heat transfer tube was sonicated in the acetone solutionfor 5 minutes and sonicated in the ethanol solution for 5 minutes. Theassembled heat transfer tube was sonicated in the acetone solution andthe ethanol solution and dipped in 2 M of hydrochloric acid (HCL) for 30seconds.

The heat transfer tube made of copper and dipped into the hydrochloricacid was dipped into the rare-earth coating solution for 30 minutes.

(4) Hydrocarbon Contamination Step

1-octadecene of 25 ml/m³ or more was put in the airtight container andthe heat transfer tube dipped into the rare-earth coating solution ofthe (2) was put therein. The airtight container was heated in the ovenat a temperature of 40 to 60° C. for 6 hours.

Experimental Example 1

Comparison of Difference in Coating State Depending on Concentration ofCe(NO₃)₃

TABLE 1 Content range Example 2 7 wt % Example 3 9 wt % ComparativeExample 1 3 wt % Comparative Example 2 10 wt % 

To compare the difference in the coating state depending on aconcentration of ≈Ce(NO₃)₃, the rare-earth coating solution was preparedby making the concentration of Ce(NO₃)₃ different as follow. In moredetail, the rare-earth coating solution has a powder form and is changedto CeO₂ by a chemical reaction with other solutions and is deposited onthe surface of the heat transfer tube in the changed CeO₂ state. To findout an optimal concentration range of Ce(NO₃)₃, the coating differencewas observed by fixing the peroxide to 1.3 wt % and changing Ce(NO₃)₃from 1 wt % to 15 wt % by 1 wt %. When a solvent is the de-ionizedwater, if the accurate quantity of Ce(NO₃)₃ added to the solution is Awt %, it may be calculated by A×1000÷(100−A) g. That is, the coatingstate was confirmed while the quantity of Ce(NO₃)₃ is changed from 10.1g to 176.47 g.

TABLE 2 Advanced Stop Receding contact angle contact angle contact angle(°) (°) (°) Comparative 3 wt % 158.4 ± 0.7 152.5 ± 0.8 <10 Example 1Example 1 4 wt % 162.6 ± 0.8 163.9 ± 4.4 161.3 ± 5.2 Example 2 7 wt %161.0 ± 0.7 158.7 ± 0.5 157.6 ± 3.2 Example 3 9 wt % 162.4 ± 1.7 161.8 ±3.0 156.4 ± 1.2 Comparative 10 wt %  159.9 ± 0.7 157.4 ± 1.0 <10 Example2

It was confirmed from FIG. 8 and the result of the above Table 2 that anoptimal quantity of Ce(NO₃)₃ ranges from 4 to 9 wt % (41.68 g to 98.9g). As shown in the above Table 2, the contact angles between 4 wt % and9 wt % corresponding to the minimum/maximum quantity of the optimalrange, the contact angle at 7 wt % included in the range, and thecontact angles at 3 wt % and 10 wt % out of the range were measured. Allthe contact angles well appeared as about 160° within the correspondingrange but as could be appreciated in FIG. 8, the receding contact anglesof 3 wt % and 10 wt % out of the range were measured as less than 10°.That is, if the contact angle is out of the optimal range of Ce(NO₃)₃,the phenomenon that a drop of water is pinned occurs, which may bedetermined that the coating is not uniformly made.

Experimental Example 2

Comparison of Difference in Coating State Depending on Concentration ofH₂O₂

TABLE 3 Content range Example 4 1.5 wt % Example 5 2.0 wt % ComparativeExample 3   1 wt % Comparative Example 4 2.3 wt %

The H₂O₂ is generally a solution used to cause a catalytic reaction andwas used to change Ce(NO₃)₃ to CeO₂ using property having strongoxidizing power. To find out the optimal concentration range of H₂O₂,the coating difference was observed by fixing the Ce(NO₃)₃ to 4 wt % andchanging the concentration of H₂O₂ from 0.5 wt % to 2.5 wt % by 0.25 wt%. The used H₂O₂ solution is a 35% diluted solution and the coatingstate was confirmed while being changed from 5.02 mL to 52.63 mL.

TABLE 4 Advanced Stop Receding contact angle contact angle contact angle(°) (°) (°) Comparative   1 wt % 147.1 ± 2.9 148.1 ± 4.1 <10 Example 3Example 1 1.3 wt % 162.4 ± 1.5 161.5 ± 2.7 161.7 ± 3.4 Example 4 1.5 wt% 158.9 ± 3.0 158.9 ± 2.7 158.3 ± 3.7 Example 5 2.0 wt % 159.5 ± 0.9159.4 ± 1.5 158.3 ± 3.6 Comparative 2.3 wt % 159.9 ± 0.7 159.4 ± 3.0 <10Example 4

Referring to the above Table 4 and FIG. 9, the range of an optimalquantity of H₂O₂ was confirmed as 1.3 wt % to 2.0 wt % (25.64 mL to41.67 mL). As shown in the above Table 4, the contact angle between 1.3wt % and 2.0 wt % corresponding to the minimum/maximum quantity of theoptimal range, the contact angle at 1.5 wt % included in the range, andthe contact angle at 1 wt % and 2.3 wt % out of the range were measured.All the contact angles well appeared as about 160° within thecorresponding range but as could be appreciated in the above Table 4 andFIG. 9, the receding contact angles of 1 wt % and 2.3 wt % out of therange were measured as less than 10°. That is, if the contact angle isout of the optimal range, the phenomenon that a drop of water is pinnedoccurs, which may be determined that the coating is not uniformly made.

Experimental Example 3

Comparison of Difference in Coating State Depending on Dipping Time ofHeat Transfer Tube Made of Copper

TABLE 5 Dipping time Example 6 20 minutes Example 7 40 minutesComparative Example 5 10 minutes Comparative Example 6 50 minutes

When a copper surface is put in a solution prepared with 4 wt % ofCe(NO₃)₃, 1.3 wt % of H₂O₂, and 1000 mL of de-ionized (DI water), thechange in the coating state was confirmed over time. The change in thecoating state was confirmed by taking out the copper surface every 5minutes from 5 minutes to 60 minutes.

TABLE 6 Advanced Stop Receding contact angle contact angle contact angle(°) (°) (°) Comparative 114.9 ± 1.6 110.6 ± 0.7 <10 Example 5 Example 6159.5 ± 0.5 158.3 ± 1.2 154.9 ± 0.3 Example 1 162.4 ± 1.5 161.5 ± 2.7161.7 ± 3.4 Example 7 160.6 ± 0.3 160.7 ± 0.4 159.3 ± 1.5 Comparative149.4 ± 1.7 150.8 ± 2.7 <10 Example 6

Referring to the above Table 6 and FIG. 10, an optimal time range of thecopper surface was confirmed as 20 to 40 minutes. As shown in the aboveTable 6, the contact angles at 20 minutes and 40 minutes correspondingto the minimum/maximum time of the optimal time range, the contact angleat 30 minutes included in the range, and the contact angles at 10minutes and 50 minutes out of the range were measured. All the contactangles well appeared as about 160° within the corresponding range, butas could be appreciated in the above Table 6 and FIG. 10, the recedingcontact angles of 10 minutes and 50 minutes out of the range weremeasured as less than 10°. That is, if the contact angle is out of theoptimal range, the phenomenon that a drop of water is pinned occurs,which may be determined that the coating is not uniformly made.

Experimental Example 4

Comparison of Difference in Coating State Depending on Dipping Time ofHeat Transfer Tube Made of Aluminum

TABLE 7 Dipping time Example 8 30 minutes Example 9 60 minutes Example10 120 minutes Comparative Example 7 5 minutes Comparative Example 8 180minutes

When an aluminum surface is put in a solution prepared with 4 wt % ofCe(NO₃)₃, 1.3 wt % of H₂O₂, and 1000 mL of de-ionized (DI water), thechange in the coating state was confirmed over time. The change in thecoating state was confirmed by taking out the aluminum surface every 30minutes from 5 minutes to 180 minutes.

TABLE 8 Advanced Stop Receding contact angle contact angle contact angle(°) (°) (°) Comparative 151.5 ± 2.6 148.8 ± 1.2 <10 Example 7 Example 8161.0 ± 1.6 161.1 ± 1.1 158.3 ± 2.1 Example 9 161.2 ± 3.2 161.0 ± 1.9160.1 ± 1.8 Example 10 162.6 ± 0.8 163.9 ± 4.4 161.3 ± 5.2 Comparative157.3 ± 1.6 156.1 ± 0.8 <10 Example 8

Referring to the above Table 8 and FIG. 11, an optimal time range of thealuminum surface was confirmed as 30 to 120 minutes. As shown in theabove Table 8, the contact angles at 30 minutes and 120 minutescorresponding to the minimum/maximum time of the optimal time range, thecontact angle at 60 minutes included in the range, and the contactangles at 5 minutes and 150 minutes out of the range were measured. Allthe contact angles well appeared as about 160° within the correspondingrange, but as could be appreciated in the above Table 8 and FIG. 11, thereceding contact angles of 5 minutes and 180 minutes out of the rangewere measured as less than 10°. That is, if the contact angle is out ofthe optimal range, the phenomenon that a drop of water is pinned occurs,which may be determined that the coating is not uniformly made.

Experimental Example 5

Component Analysis Result for Coating Layer of Heat Transfer Tube HavingSuperhydrophobic Surface

The analysis result of the EDS result for the heat transfer tube ofExamples 1 and 10 is the following Table 9 and 10.

TABLE 9 Element Weight % Atomic % C 5.3 28.06 Cu 53.07 53.06 Ce 41.6318.88 Totals 100

TABLE 10 Element Weight % Atomic % C 12.31 41.92 Al 26.55 40.24 Ce 61.1317.84 Totals 100

The above Table 9 dips the heat transfer tube made of copper in therare-earth coating solution to form the coating layer, and as the resultof measuring the EDS for the heat transfer tube, may confirm that a Ceelement that is rare-earth metal to be coated is deposited and C isdeposited due to the hydrocarbon contamination step.

Similarly, the above Table 10 dips the heat transfer tube made ofaluminum in the rare-earth coating solution to form the coating layer,and as the result of measuring the EDS for the heat transfer tube, mayconfirm that a Ce element that is rare-earth metal to be coated isdeposited and C is deposited due to the hydrocarbon contamination step.

Experimental Example 6

Assessment Result of Thermal Stability

As illustrated in FIG. 12, to compare the difference in the condensationbehavior of the sample coated by the method different from the samplecoated with rare-earth in the harsh environment, the experiment wasperformed by the simple configuration. The experiment was planned tomanufacture an acrylic chamber 500 and attach a cold plate to a backsurface of the acrylic chamber 500 to control the surface temperature tobe about 28° C., attach a large capacity of beaker 600 to a bottomsurface thereof and boil water in the beaker using a hot plate 700 todirectly supply hot steam into the acrylic chamber 500 and directlycollect drops of water generated due to the condensation(S^(≈)25.30). Amore detailed manufacturing method thereof is as the following Table 11.

TABLE 11 Comparative The copper sample was manufactured by being dippedinto acetone and ethanol, Example 9 respectively, sonicated for 5minutes, and dipped into 2M of HCl for 30 seconds, washed with thede-ionized water (DI water), and dried with nitrogen gas. ComparativeThe aluminum sample was manufactured by being dipped into acetone andethanol, Example 10 respectively, sonicated for 5 minutes, and dippedinto 2M of HCl for 30 seconds, washed with the de-ionized water (DIwater), and dried with nitrogen gas. Comparative The copper sampletreated as shown in the Comparative Example 9 was Example 11manufactured by being put in the airtight container together with asolution formed by mixing toluene and heptadeca-fluoro-1,1,2,2,2tetrahydrodecyl trichlorosilane (HDFS) solution at 20:1 and putting theairtight container in an oven of 85° C. for 3 hours. Comparative Thealuminum sample treated as shown in the Comparative Example 10 wasExample 12 manufactured by being put in the airtight container togetherwith a solution formed by mixing toluene and heptadeca-fluoro-1,1,2,2,2tetrahydrodecyl trichlorosilane (HDFS) solution at 20:1 and putting theairtight container in an oven of 85° C. for 3 hours.

TABLE 12 Advanced contact angle (°) Stop contact angle (°) Recedingcontact angle (°) Before After 24 Before After 24 Before After 24experiment hours experiment hours experiment hours Comparative  84.1 ±1.7 62.3 ± 6.5  78.4 ± 1.4 60.4 ± 7.3 30.8 ± 3.5 <10 Example 9Comparative  92.8 ± 0.1 36.6 ± 1.7  86.8 ± 1.6 35.1 ± 4.9 26.4 ± 3.4 <10Example 10 Comparative 122.5 ± 2.0 78.1 ± 1.1 116.3 ± 5.6 77.3 ± 1.573.1 ± 5.6  48.8 ± 3.5 Example 11 Comparative 123.2 ± 3.4 51.2 ± 4.5122.6 ± 3.4 32.1 ± 5.3 82.2 ± 5.4 <10 Example 12 Example 1 162.4 ± 1.5160.8 ± 1.6  161.5 ± 2.7 159.7 ± 0.8  161.7 ± 3.4  158.7 ± 2.7 Example10 162.6 ± 0.8 146.1 ± 1.5  163.9 ± 4.4 139.2 ± 0.1  161.3 ± 5.2  139.5± 4.8

FIGS. 13 to 15 are photographs of experiment results of measuringthermal stability, in which the top two photographs of FIG. 13 relate tothe Comparative Example 9 and the bottom two photographs relate to theComparative Example 10. The above two photographs of FIG. 14 relate toComparative Example 11 and the bottom two photographs relate to theComparative Example 12. The above two photographs of FIG. 15 relate tothe Example 1 and the bottom two photographs relate to the Example 10.

Referring to FIGS. 13 to 15 and the above Table 12, the thermalstability of the Comparative Examples in which the existing repellentcoating is performed and the thermal stability of the Example in whichthe rare-earth coating is performed were performed. FIG. 13 illustratesthe thermal stability experimental results for the Comparative Examples(the above two drawings) and 10 (the bottom two drawings), in which thecopper (Comparative Example 9) and aluminum (Comparative Example 10)samples do not have the coating layer unlike other samples, andtherefore a result of reducing the contact angle of the surface appearedover time. Unlike this, in the case of the Comparative Example 11 and 12and the Examples 1 and 10 manufactured by the existing repellent coatingmethod, all the condensation behaviors in a dropwise form appearinitially but the repellent surface is changed to a filmwise form whilethe behavior gradually collapses and thus a result of reducing thecontact angle appeared after 24 hours lapse. However, in the case of theExamples 1 and 10, it was confirmed that the condensation behavior inthe dropwise form is still maintained even after 24 hours lapse.

For more quantitative confirmation, the contact angle of the samplebefore/after the thermal stability experiment was measured as the aboveTable 12. Unlike the Examples 1 and 10 in which the contact anglebefore/after the experiment is maintained to some extent, theComparative Examples 11 and 12 confirmed that the contact angle isremarkably reduced after the experiment. It is determined that therepellent coating of the existing repellent surface disappears comparedto the rare-earth that well withstands the high temperature environmentand it is determined that roughness of the sample itself is also reducedon the basis of the result of reducing the contact angle of the copperand aluminum samples.

Experimental Example 7

Measurement of Change in Contact Angle Due to Difference in Form

TABLE 13 Comparative It was manufactured in a pellet form by compressingceria powder in a Example 13 cylindrical steel press frame at 270 MPafor 3 minutes and then compressing it at 350 MPa for 5 minutes, andsintering it at 1560° C. for four hours. Comparative It was manufacturedby depositing a thin (~200 to 350 nm) rare-earth layer Example 14 usinga sputtering scheme. Comparative It was manufactured by dipping the heattransfer tube made of copper in the Example 15 solution having 4 wt % ofCe(NO₃)₃, 0.2 vol % of H₂O₂, 0.2 mM of NaCl, and HNO3 (pH = 3.5).

FIGS. 16 to 20 are the comparison photographs for measuring the changein contact angle due to the difference in form. FIG. 16 relates to theComparative Example 13, FIG. 17 relates to the Comparative Example 14,FIG. 18 relates to the Comparative Example 15, FIG. 19 relates to theExample 1, and FIG. 20 relates to the Example 10.

Referring to FIG. 16, it may confirm that the Comparative Example 13 hasvery low roughness. There is a limitation in that the contact angle maynot be increased up to about 102° due to the roughness. Further, asshown in the above Comparative Example 14 of FIG. 17, even when it ismanufactured by a sputter, the roughness is reduced, and thus thetextured structure is manufactured and then the contact angle of thesuperhydrophobic water to be coated may be increased to about 160°.Referring to the Comparative Example 15 of FIG. 18, it may beappreciated that high roughness may be formed only by the dippingscheme. However, the Comparative Example 15 generates a huge number ofcracks are and thus when being applied to the heat transfer tube for theactual condenser and heat exchanger, may cause the pinning phenomenon.On the other hand, the Examples 1 and 10 may more simply and cheaplymanufacture the uniform rare-earth oxide coating layer and has the highcontact angle due to the high roughness.

Experimental Example 8

Increase in Thermal Resistance Due to Difference in Coating Thickness

FIGS. 21 to 24 are an SEM photograph and a FIB comparison photograph ofthe sample due to the difference in the manufacturing method for coatingrare-earth oxide. The Comparative Example 13 of FIG. 21 may confirm thatthe coating thickness is thick as 2 mm. The Comparative Example 16 ofFIG. 22 is manufactured in the pellet form like the Comparative Example13 and then additionally perform irradiation and may confirm as havingthe coating thickness similar to the above Comparative Example 13. Thethick thickness causes the increase in thermal resistance when it isapplied to the actual heat transfer tube for the condenser and the heatexchanger, thereby causing the loss in the heat transfer.

On the other hand, referring to the Example 1 of FIG. 23 and the Example10 of FIG. 24, it may be confirmed that the average coating thickness isvery thin as 200 nm and the difference in the coating thickness of about10,000 times shows the difference in about 80 times in the theoreticalcondensation heat transfer performance.

According to the heat transfer tube having rare-earth oxide deposited onthe surface thereof and the method for manufacturing the same of thepresent disclosure, the rare-earth oxide can be deposited on the surfaceof the heat transfer tube to implement the superhydrophobic surface evenunder the high temperature environment and the plurality of assembledheat transfer tubes can be coated by coating the complex shape bydepositing rare-earth oxide using the method for dipping a surface ofthe heat transfer tube and coating the same, thereby reducing orpreventing the heat transfer tubes from being damaged during theassembling of the heat transfer tubes after the coating.

Hereinabove, preferred exemplary embodiments of the present disclosureare described for illustrative purpose, and the scope of the presentdisclosure is not limited to the above described specific exemplaryembodiment. It will be apparent to those skilled in the art that variousvariations and modifications may be made without departing from thespirit and scope of the disclosure as defined in the following claims.

What is claimed is:
 1. A heat transfer tube made of copper (Cu), theheat transfer tube comprising: a coating layer formed on a surface ofthe heat transfer tube as a superhydrophobic surface of the heattransfer tube, the coating layer including a rare-earth oxide, whereinthe coating layer further includes a deposition layer of carbon (C),wherein the rare-earth oxide is formed using a rare-earth coatingsolution into which the heat transfer tube is dipped for 20 to 40minutes, and wherein the rare-earth coating solution gives a compositionof the coating layer of 5.30 wt % of carbon (C), 53.07 wt % of copper(Cu), and 41.63 wt % of an oxide of cerium (Ce), the oxide of cerium(Ce) serving as a substrate for the deposition layer of carbon (C). 2.The heat transfer tube of claim 1, wherein a thickness of the coatinglayer without the deposition layer of carbon (C) ranges from 100 to 400nm.
 3. The heat transfer tube of claim 1, wherein the deposition layerof carbon (C) is formed by hydrocarbon contamination of the coatinglayer without the deposition layer of carbon (C), and wherein thedeposition layer of carbon (C) forms the superhydrophobic surface of theheat transfer tube.
 4. The heat transfer tube of claim 3, wherein therare-earth oxide of the coating layer includes CeO₂ to which thehydrocarbon contamination is performed.
 5. A heat transfer tube made ofaluminum (Al), the heat transfer tube comprising: a coating layer formedon a surface of the heat transfer tube as a superhydrophobic surface ofthe heat transfer tube, the coating layer including a rare-earth oxide,wherein the coating layer further includes a deposition layer of carbon(C), wherein the rare-earth oxide is formed using a rare-earth coatingsolution into which the heat transfer tube is dipped for 30 to 120minutes, and wherein the rare-earth coating solution gives a compositionof the coating layer of 12.31 wt % of carbon (C), 26.55 wt % of aluminum(Al), and 61.13 wt % of an oxide of cerium (Ce), the oxide of cerium(Ce) serving as a substrate for the deposition layer of carbon (C). 6.The heat transfer tube of claim 5, wherein a thickness of the coatinglayer without the deposition layer of carbon (C) ranges from 100 to 400nm.
 7. The heat transfer tube of claim 5, wherein the deposition layerof carbon (C) is formed by hydrocarbon contamination of the coatinglayer without the deposition layer of carbon (C), and wherein thedeposition layer of carbon (C) forms the superhydrophobic surface of theheat transfer tube.
 8. The heat transfer tube of claim 7, wherein therare-earth oxide of the coating layer includes CeO₂ to which thehydrocarbon contamination is performed.